The present invention relates to a plated steel wire that exhibits high corrosion resistance suitable for steel wires for gabion, fishnets and the like that are used in areas exposed to the outdoors.
Commonly used plated steel wires include zinc-plated steel wires and the more highly corrosion-resistant zinc-aluminum alloy-plated steel wires. Zinc-aluminum alloy-plated steel wires are generally produced by first subjecting steel wires to a cleaning treatment such as washing and degreasing and then to a flux treatment, followed by either hot-dip plating of mainly zinc as the first stage and then hot-dip plating in a Znxe2x80x94Al alloy bath containing 10% Al as the second stage, or else direct hot-dip plating in a Znxe2x80x94Al alloy bath containing 10% Al, and finally vertically drawing the wire out from the plating bath, cooling and winding.
Such zinc-aluminum alloy-plated steel wires have satisfactory corrosion resistance, but even higher corrosion resistance can be achieved by methods that increase the plating thickness. One method of guaranteeing the prescribed plating thickness is a method of raising the conveying speed (flux) of the steel wire to rapidly draw out the steel wire from the plating bath, and increasing the amount of plating alloy adhering to the steel wire by increasing the viscosity of the hot-dip plating alloy.
In this method, however, the high conveying speed tends to produce an irregular plating thickness in the cross-section perpendicular to the lengthwise direction of the plated steel wire, and limits therefore exist for such plating equipment. As a result, existing plating equipment has not provided sufficient corrosion resistance by zinc plating or by hot-dip plating with Znxe2x80x94Al alloys, and this constitutes a problem in that expectations cannot be completely satisfied given current expectations regarding a longer usable life for plated steel wires.
In order to combat this problem, Japanese Unexamined Patent Publication HEI No. 10-226865 proposes a Znxe2x80x94Alxe2x80x94Mg alloy plating composition with high corrosion resistance imparted by Mg added to the plating bath, but the plating method based on this plating composition assumes thin layering for steel sheets, and when the method is applied to thick plated steel wires typically used for gabion and the like, the problem of plating layer cracking occurs when working the plated steel wires.
Japanese Unexamined Patent Publication HEI No. 7-207421 describes a method in which a Znxe2x80x94Alxe2x80x94Mg alloy plating is formed to a greater thickness, but when the method is directly applied to plating of steel wires, the Fexe2x80x94Zn alloy layer becomes thick, leading to problems such as cracking or peeling of the alloy layer when working the plated steel wires.
In light of the problems described above, it is an object of the present invention to provide a plated steel wire coated with a molten zinc alloy plating such that the plated steel wire exhibits excellent corrosion resistance and excellent workability that can avoid cracking or peeling of the plating layer and/or the plating alloy layer during working of the plated steel wire, as well as to provide a process for its manufacture.
The present invention has been completed as a result of much diligent research, by the present inventors, on a means of solving the aforementioned problems, and its gist is as follows.
(1) A plated steel wire with high corrosion resistance and excellent workability, the plated steel wire being characterized in that the average composition of the plating alloy contains, in terms of weight percentage, Al: 4-20%, Mg: 0.8-5% and the remainder Zn, and in that an Fexe2x80x94Zn alloy layer of no greater than 20 xcexcm thickness is present at the plating-base metal interface.
(2) A plated steel wire with high corrosion resistance and excellent workability according to (1) above, characterized in that the average composition of the plating alloy also contains, in terms of weight percentage, Si: xe2x89xa62%.
(3) A plated steel wire with high corrosion resistance and excellent workability according to (1) or (2) above, characterized in that the average composition of the plating alloy also contains, in terms of weight percentage, Na: 0.001-0.1%.
(4) A plated steel wire with high corrosion resistance and excellent workability according to any one of (1) to (3) above, characterized in that the average composition of the plating alloy also contains, in terms of weight percentage, Ti: 0.01-0.1%.
(5) A plated steel wire with high corrosion resistance and excellent workability according to any one of (1) to (4) above, characterized in that the Fexe2x80x94Zn alloy layer contains Al: xe2x89xa74%, Mg: xe2x89xa71%.
(6) A plated steel wire with high corrosion resistance and excellent workability according to any one of (1) to (5) above, characterized in that the structure of the plating alloy layer on the outer side of the Fexe2x80x94Zn alloy layer includes an xcex1 phase composed mainly of Alxe2x80x94Zn, a xcex2 phase comprising a Zn monophase or an Mgxe2x80x94Zn alloy phase, and a Zn/Al/Znxe2x80x94Mg three component eutectic phase.
(7) A plated steel wire with high corrosion resistance and excellent workability according to any one of (1) to (6) above, characterized in that the structure of the plating alloy layer on the outer side of the Fexe2x80x94Zn alloy layer includes an xcex1 phase composed mainly of Alxe2x80x94Zn, a xcex2 phase comprising a Zn monophase or an Mgxe2x80x94Zn alloy phase, and a Zn/Al/Znxe2x80x94Mg three component eutectic phase, and the volume fraction of the xcex2 phase is no greater than 20%.
(8) A plated steel wire with high corrosion resistance and excellent workability according to any one of (1) to (5) above, characterized in that the structure Dirt of the plating alloy layer on the outer side of the Fexe2x80x94Zn alloy layer is a dendritic structure.
(9) A plated steel wire with high corrosion resistance and excellent workability according to any one of (1) to (5) above, characterized in that the structure of the plating alloy layer on the outer side of the Fexe2x80x94Zn alloy layer is a granular crystal structure.
(10) A plated steel wire with high corrosion resistance and excellent workability according to any one of (1) to (9) above, characterized in that the component composition of the plated steel wire comprises, in terms of weight percentage, C: 0.02-0.25%, Si: xe2x89xa61%, Mn: xe2x89xa60.6%, P: xe2x89xa60.04% and S: xe2x89xa60.04%.
(11) A process for the manufacture of a plated steel wire with high corrosion resistance and excellent workability, characterized in that the process for manufacture of a plated steel wire comprises coating a steel wire with a molten zinc plating composed mainly of zinc as the first stage, and then coating it with a molten zinc alloy plating having the average composition specified in any one of (1) to (4) above as the second stage.
(12) A process for manufacture of a plated steel. wire with high corrosion resistance and excellent workability according to (11) above, characterized in that the molten zinc plating as the first stage is a molten zinc plating comprising, in terms of weight percentage, Al: xe2x89xa63% and Mg: xe2x89xa60.5%.
(13) A process for the manufacture of a plated steel wire with high corrosion resistance and excellent workability according to (11) or (12) above, characterized in that in the steps of coating with a molten zinc plating as the first stage and coating with a molten zinc alloy plating as the second stage, the part of the plated steel wire drawn out from the plating bath is purged with nitrogen gas to prevent oxidation of the bath surface and the plated steel wire.
(14) A process for the manufacture of a plated steel wire with high corrosion resistance and excellent workability according to any one of (11) to (13) above, characterized in that the molten zinc plating as the first stage is coated for a maximum plating bath immersion time of 20 seconds, and the molten zinc alloy plating as the second stage is coated for a maximum plating bath immersion time of 20 seconds.
(15) A process for the manufacture of a plated steel wire with high corrosion resistance and excellent workability according to any one of (11) to (14) above, characterized in that in the steps of coating with a molten zinc plating as the first stage and coating with a molten zinc alloy plating as the second stage, the wire is directly cooled by a water spray, steam or a water flow immediately after the plated steel wire is drawn out from the plating alloy, to harden the plating alloy.
(16) A process for the manufacture of a plated steel wire with high corrosion resistance and excellent workability according to any one of (11) to (15) above, characterized in that in the steps of coating with a molten zinc plating as the first stage and coating with a molten zinc alloy plating as the second stage, the initial cooling temperature for cooling of the plated steel wire is in a range from the melting point of the plating alloy to 20xc2x0 C. above the melting point.
(17) A process for the manufacture of a plated steel wire with high corrosion resistance and excellent workability according to any one of (11) to (16) above, characterized in that the component composition of the plated steel wire comprises, in terms of weight percentage, C: 0.02-0.25%, Si: xe2x89xa61%, Mn: xe2x89xa60.6%, P: xe2x89xa60.04% and S: xe2x89xa60.04%.